If you see files downloaded from Dropbox to your computer that were previously online-only, and you didn’t open or change the files, another application on your computer might be causing the issue. Some applications need to look at your files to work, and that makes Dropbox download the files.
Some examples of applications that do this are:
If you see this happening, you can make the file online-only again. To make a file online-only:
You can also determine which application is causing the issue, then disable the application or adjust its settings. To do this, quit the applications running on your computer one by one. When you see the issue resolved, the last application you quit is likely the cause of the issue.
Note: Keep in mind Dropbox can't control the other applications on your computer. When you determine which application was causing the problem, you may have to disable it, or change its settings if possible, to prevent the issue.